Filament structure



April 1, 1958 B. K. M. MAGNUSSON 2,829,297 FILAMENT STRUCTURE Fil ed April 15} 1954 2 Sheets-Sheet 1 24 27 27 l 1' G J 35: c 29 I3 IO 35 INVENTOR. BROR KM. MAGNUSSON April 1, 1958 B. K. M. MAGNUSSON FILAMENT STRUCTURE 2 Sheets-Sheet 2 Filed April 15. 1954 FIG. 8

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INVENTOR.

BROR K.M.MAGNUSSON BY 6 7 ATTORNEY United States Patentt) 2,829,297 FILAMENT STRUCTURE Bror K. M. Magnusson, Stamford, Conn., assignor to Machlett Laboratories, Incorporated, Springdale, Conn., a corporation of Connecticut Application April 15, 1954, Serial No. 423,421

' 6 Claims. (Cl. 313279) cidence of breakage during handling and transportation.

My experiments have indicated that stresses introduced in the filaments while cooling down from high temperatures probably account for the large amount of breakage.

Although tungsten wire as a product of manufacture is ductile, it is well known that filaments made from this wire become brittle due to the heating processes encountered during the manufacture of electron tubes and during their service. This embrittlement occurs to some extent inpure tungsten filaments but to a much greater extent in thoriated'tungsten filaments due to their carburization.

Filament breakage seems to occur most frequently at points intermediate the supports, rather than .at the supports. In most tube structure experiencing ahigh amount of filament breakage, this intermediate portion of the filament is either, straight or else has very little curvature. According to my analysis embrittling stresses must be introduced because of bending and/ or unbending? of these straight portions of the filament, and the amount of bending or unbending would accordingly seem to account for the amount of brittleness experienced.

, It can readily be shown that a straight wire supported at each end will bend considerably more than asimilarly supported curved wire due to expansion when each wire is heated to the same high temperature (i. e., the

relative change in radius of curvature will be greater in thestraight wire). It is a well known physical fact that the greater is the bending, the greater the stresses introduced into the filament. .Of course, stresses introduced during bending are relieved to a large extent at the filaments high operating temperature. When the filament is. cooled, however, the'resulting straightening or unbending will introduce stresses which can be only partly relieved due to therapid decrease. in temperature.

Consequently stresses, which effectively weaken the filament so that it will break more easily, remain in the wire after cooling. Unquestionably, filaments do not usuallybreak due to these stresses alone. However, a filament with no or a small. amount of remaining stresses will obviously withstand more severe additional stresses dueto handling and transportationwithout breaking than a semester the same inherent brittleness which has a considerable amount of such remaining stresses.

In accordance with the present invention stresses in a filamen't due to expansion and contraction effects, as described above, are reduced by making, the various porti'ons between supportscurved.- Asimentioned before,

a curved filament portion supported at each end will 2,829,297 Patented Apr. 1, 1958 2 1 bend and unbend less than a similarly supported straight filament portion when heated and cooled. In fact, it is desirable that each segment of the filament have sufiicient curvature to deviate substantially from a straight line drawn between the two filament supports. The angular segment of the filament between two supports (i. e., the angular displacement of the supports, in the case of coaxial circular segments) is, accordingly, preferably made as large as possible. True, other factors influence the hot to cold curvature ratio, such as axial length of the filament, the diameter of the cylinder defined -by filament, etc. However, since these factors usually must be determined by other design considerations, the angular displacement of filamentsupport memangular segment employed between filament supports the better, other factors being equal. Actually, it is true that helical portions of the filament supported at points an'gularlydisplaced 180 from one anotherare more rugged than those having angular supports only apart. In fact, it is true that the greater the angular displacement of supports, the less likely that breakage will occur. Beyond however, the factor of stability assumes great importance. If filament strands were not somewhat rigid, and consequently self-supporting, it would not be possible for them to be made to extend through'more than 180. In fact, even asrigid as they are,-should a portion start to distort away from its initial position, the electrostatic pull on that portion exerted by the anode would tend to further distort it, and this pull would increase as distortion progressed. However,

the rigidity of filaments makes it possible for them to be supported at points more than 180 apart. In fact, points separated 270 around'an axis have been suc cessfully used on occasion From a manufacturing point of view it is convenient to specify the shape of the filament in terms of theangular displacement of the supports around the axis of the filament cylinder. In predicting the stability of the filament, however, it is advisable to examine each portion of the filament under consideration in relation to a plane through its two supports and through the midpoint of said portion. gular support displacement of 180 around the filaments axis is not necesesarily critical, unless the axial length of the filament is very short.

and its stability, accordingly, becomes moreand more dependent upon the rigidity of the wire and the forces acting upon it. The rigidity, in turn, depends upon wire diameter, temperature, etc., and the forces depend upon filament to anode voltage, filament to anode distance, etc. Thus a definite limit cannot be defined without, in each case, defining such other factors. However, in most instances where helical portions are used, it is advisable upon cooling in the supported helical portions. The helirun thejsame' direction but may be of different pitch. T here must be at least two pitches employed and adjacent.

it will thus be seen that an an-- It will also be seen, however, thatbeyond.180., a helical curve deviates increaslngly from the plane through its support and midpoint,

, In a filament having a plurality of helices with different,

pitch, it is possible for a single filament strand to be em.- ployed even though all the helices have the same direction.. The use of one piece to make a multi-section filament offers an important advantage. Any filaments temperature in service is usually determined by ndjusting'thc filament voltage to certain specified values. If the resistance is lower than it should be, the current and consequently the temperature will be too high, and the filament life may be needlessly shortened. If the resistance is too high the temperature will be too low and the electron emission may be insufficient for proper functioning. Thus, the filament resistance should not deviate much from the proper value. Small unpredicted deviations may be caused at the current carrying joints between the filament and the supports if the points of contact are displaced slightly from the prescribed position or if the contact resistances are different fromone tube to another. In the present invention by making the filament from a single piece of wire, the number of such joints has been reduced to a minimum. Hence, the filament of my invention under the practices commonly experienced in the use of tubes will insure uniform performance and long life of tubes in which it is employed.

The present inventionin its preferred form is a structure which takes advantage of the factors previously alluded to and which is quite stable. It is one which employs a single strand filament having three helical por tions separated by two bights. It is so arranged that it defines a cylinder such that one of the bights and one of the ends of the filament terminate at one end of the cylinder and the other bight and the other end of the filament terminate at the other end of the cylinder. This filament structure is supported atits ends and at the bights. In the preferred case, the middle helical portion is also supported at its midpoint. More specifically, the preferred form of this structure has helical portions adjacent the ends, which portions extend between support members displaced about 180 around the axis of the cylinder, and the middle portion has its supports at the bights spaced apart 360 around the axis of the cylinder and is supported at the midpoint. In many cases it has been found advantageous to make these helical portions somewhat longer so that they extend to morethan 180 and more than 360, respectively, around the axis of the cylinder defined. Another favored construction permits any angular displacement within the permissible ranges defined, provided that the length of the middle helical portion is approximately twice as long as the length of the respective end portions. It should be pointed out, however, that I even thoughthc preferred constructions take that form. there is, of course, no requirement that the middle portion be twice as long as the end portions in order to practice the present invention. In fact, the middle portion may be any length relative to the end portions.

Supports for the filament structure advantageously extend laterally from rod-like support members which are embedded in and may extend through the stem press. These rod-like members are advantageously all made to lie in one plane, said plane passing through the axis of the cylinder defined by the filament.

This invention also contemplates the use of more than three helical portions in a continuous filament. Such a construction might be valuable where heavier electron loading over an anode of given area is required. In such event all of the helical portions would have the same direction around the cylindrical surface defined but at least two different pitches would have to be employed.

supports separntcd by approximately 120 and 240,

iii

spectively, might be employed. Various other patterns are possible and might be useful in specific applications. Of course, the more helical portions employed, the more supports required and the more complex the structure becomes.

For a better understanding of this invention reference is made to the following drawings:

Fig. l is a side elevational view of the preferred filament structure of the present invention.

Fig. 2 is a plan view from the bottom of Fig. 1.

Fig. 3 is an elevational view of the same structure as Fig. 1 taken at from the position shown in Fig. 1.

Fig. 4 is a plan view corresponding to Fig. 2 but rotated 90 and corresponding to the position shown in Fig. 3.

5 is a plan view of the filament wire alone.

Fig. 6 is a side elevational view of the filament wire alone.

Fig. 7 is another side elevational view of the same filament wire rotated on its axis 90 from the position shown in Fig. 6.

Fig. 8 is a schematic representation of the filament of Figs. 5-7 as it would appear if rolled out into a plane.

Fig. 9 is a schematic representation similar to that of Fig. 8 but representing a single strand filament having five helical portions.

Referring now to the drawings, and particularly to Figs. 5, 6 and 7 which show the preferred form of the filament alone and unsupported, it can be seen that the filament in its preferred form is a single strand. The filament as shown in Fig. 6 is in a position corresponding to that shown in Fig. 3. The filament as shown in Fig. 7 is in a position rotated 90 about the axis of the cylinders generated by said filament. The view in Fig. 7 is also rotated 180 from the view shown in Fig. 1. As may be readily seen in Figs. 6 and 7, the filament consists of three helical portions 10, 11 and 12. Between these helical portions are bights 13 and 14. The helical portions are arranged to define a generally cylindrical surface and to provide a very uniform electron loading of cylindrical anode structure normally employed with the filament. End 15 of the filament terminates at the same end of the cylinder as bight 13, while end 16 of the filament terminates at the other end of the cylinder where bight 14 is located.

Fig. 8 schematically illustrates the filament of Figs. 5-7 as it would appear if the cylinder which it defines were rolled out fiat. From this illustration it can be seen that all of the helical portions have the same direction but the pitches of adjacent portions differ. Thus, it is possible to use a single filament strand and yet achieve highly uniform electron loading of the anode.

Fig. 9 schematically illustrates a filament having five helical portions all having the same direction. Again it will be noted that the pitches of adjacent helical portions differ but alternate helical portions advantageously have the same pitch to preserve the advantage of uniform electron loading which is achieved from such balance in the pattern. In this instance, the supports for the helical portions at the opposite ends of the defined cylinder are spaced approximately and 240 apart around the axis of the cylinder. It is, of course, possible within the scope of this invention to have even more than five helical portions of the filament structure in order to obtain greater electron emissive area for a fixed amount of anode area. However, as the number of supports required for the filament increases, the complexity of the structure and its cost of manufacture is increased. Accordingly, it is believed that for most applications the versions of this filament having only three, four or five helican portions will be preferred.

Support for the preferred structure shown is provided at the ends of the filament strand, the bights and the midpoint of the intermediate helical portions. Support is provided at the intermediate helical portion because this portion extends through an angle of approximately 360, more or less, around the axis of the cylinder defined by the filament. Accordingly, the support at its midpoint is necessary for stability. The nature of the supports will be treated later.

Inasmuch as the filament structure in this case is supported within the vacuum envelope on a stempress, it is convenient to make this stern press the basic" mounting structurefor the whole filament! Accordingly,j as illus-r trated in Figs. 1-4, in the present invention a plurality of metallic rods are employed for this purpose. The stempress 1B terminates a reentrant part of the tube envelope 19. A metallic shield may be provided to surround said stem. Said shield may be cup-like, as is shown by shield 20, or of any convenient form. The preferred arrangement of the mounting rods which ultimately support the filament is in a single plane, said plane passing through'the axis of the cylinder defined by the filament structure. Five rods 22, 23, 24, 25 and 26, all advantageously made of tungsten, are employed. Rods 22, 24 and 26 are merely. mounting structures and as such may be embedded in the stem press, without being brought through the envelope. Shield 20 may advantageously besupported on axial rod 24 using an annular flange member 20a. Rods 23 and 25 will carry current and accordingly should pass through stem press 18 to the outside of the vacuum envelope where they may be connected to flexible leads or connections to a current source. The five rods may be rigidized and held in place relatively to one another, by, the use of a pair of dielectric blocks 27 on opposite sides of the rods. The blocks are; advantageously notched to accept each rod in its proper place and are heldin place byfine wire which binds them together and to one or more of the rods as shown. Thedielectric blocks 27 may, of course, be fixed in place in various other convenient manners. The ends of the filament 16 and 15 are respectively connected to rods 25 and 23. In the preferred arrangement of the support structure, which may best be seen in Figs. 2 and 4, the ends of the filament are supported by metallic clips mounted on rods 23 and 25. Clip 28 on rod 23 supports end 15 of the filament strand. Clip 29 on rod 25 supports end 16 of the filament strand. All other supports are advantageously coiled wire engaging the rod and having an extension laterally outwardly from the rod which extension forms a loop for support of some portion of the filament. For instance, support 31 is affixed to rod 22 and supports bight 13. Support 32 is atlixed to rod 26 and supports bight 14. Support 33 afiixed to rod 24 supports the midpoint of helical portion 11. The various supporting members 28, 29, 30, 31, 32 and 33 are all advantageously made of molybdenum. The rod members are preferably made no longer than necessary to mount their respective supports at their ends. As shown, the rods 22 and 23 are placed adjacent to one another inasmuch as their lengths may be kept approximately the same so that they may support, through support members 31 and 28, respectively, bight 13 and end 15 of the filament structure. Bight 14 and end 16 are, of course, relatively close to the stem press at the opposite end of the cylinder previously mentioned so that their support rods 26 and 25, respectively, are advantageously about the same length and positioned adjacent one another. Rod 24 which supports the midpoint of helical portion 11 is advantageously terminated about the middle of the cylinder defined by the filamentstructure and is preferably the middle rod. Using this arrangement, it is possible for additional rigidity to be given to the longer rods 22, 23, 24 by tying them together with a metallic strap 35 near the end of rod 24. The strap is advantageously affixed by welding it, or similarly aflixing it, to rod 23 and is insulated from rods 22 and 24 by tubular insulators 36 and 37, respectively.

The arrangement of supports is not of basic importance to the invention. For instance, within the scope of the present invention, the rods might be arranged at the ends of the arms and in the middle of a cross-like press. In fact, those support elements which serve only a support ing function and no electrical conducting function may be supported on any rigid, sturdy tube member, including the support-conductor rods, if proper insulation is included. A preferred pattern of rearrangement places the rods in a planar arrangement as illustrated in the drawings but employs the outside rods, corresponding to rods 22 and 26, to support end 16 and bight 14. The two intermediate rods corresponding to rods 23 and 25 support end 15 and bight 13. And the middle rod is essentially unchanged.

It will be noted that in the structure illustrated the helical portions adjacent the ends of the filament extend over approximately 180 measured around the axis of the cylinder defined by the filament between the supports. Filament portion 11, however, extends over an angle well in excess of 360 measured around the axis of the cylinder defined by the filament and the support at its midpoint divides the intermediate portion into two helical portions extending in excess of 180 between the supports measured around the axis of the cylinder. It has been found that these helical sections are quite stable,

venient to arrange and generally provides more uniform,

electron loading, but it is possible to use an even number. The angular displacement of the supports at the opposite ends of a helical portion'does not limit the number of helical portions which may be employed. The supports at the opposite ends of the defined cylinder for a particular helical portion may be Widely displaced, yet adjacent supports at the same end of the cylinder may be separated by a relatively small angle. For instance, it is possible to employ five helical portions with their supports and 240 apart as shown in Fig. 9 or with their supports and 270 apart, etc. Incidentally, the wider spacing'of supports will in many cases require more supports, e. g., in the case of the five portion filament in Fig. 9 six supports are needed whereas in the alternative form just suggested eight supports would be required. Wider angular spacing of supports for opposite ends of the helical portion gives more filament length for a given anode area so that a higher electron loading is possible, but it requires the additional two undesirable supports. Likewise, where an odd number of helical portions is employed, it is usually desirable to arrange the filament so that the longer helical portions which require an extra support member, are fewer.

It has been pointed out that a'single strand is preferred in this invention. However, if for some reason more I than one strand is employed the resulting structure must be considered as a modified form or the equivalent of the invention described it it is otherwise a structure described in this application. Many other variations in form of the present invention will also occur to those skilled in the art. This invention includes all variations and modifications of the structures described herein within the scope and spirit of the claims.

I claim:

1. A filament structure for use in a vacuum tube envelope comprising a filament strand having three portions defining a single generally cylindrical surface, all of the portions being helical in form and extending in the same direction around the cylindrical surface which they form, one of said portions beginning at one end of the structure and extending around one side thereof through an angle of at least 180 but no more than 270 and terminating at the opposite axial end of the structure, the second portion beginning at a point closely adjacent the terminal end of the first portion and having a different pitch from said first portion and extending about the same side of the cylindrical surface as the first portion, the second portion further continuing around the opposite side of the cylindrical surface and extending throughout a total angle substantially twice that of the first portion and terminating at the end of the structure opposite that where it began, and the third portion beginning at a point closely adjacent the terminal end of the second portion and having a pitch substantially the same as the first portion, the third portion extending around the side of the structure opposite the first portion and through an angle of at least 180, but no more than 270 and terminating at the end of the structure opposite the beginning point of said first portion.

2. A filament structure as set forth in claim 1 wherein the closely adjacent ends of the first and second portions are connected by an integral bight and the closely adjacent ends of the second and third portions are similarly connected by an integral bight, whereby the portions form a single continuous one-piece strand.

3. A filament structure as set forth in claim 1 wherein support for the filament is provided at the closely adjacent ends of the first and second portions and at the closely adjacent ends of the second and third portions, and at a point substantially midway of the second portion, whereby the portions are restrained from substantial expansion in directions radially of the cylindrical structure.

4. A filament structure for use in a vacuum tube envelope comprising a filament strand having three portions A defining a single generally cylindrical surface, all of the portions being helical in form and extending in the same direction around the cylindrical surface which they form, one of said portions beginning at one end of the structure and extending around one side thereof through an angle of about 180 and terminating at the opposite axial end of the structure, the second portion beginning at a point closely adjacent the terminal end of the first portion and having a different pitch from said first portion and extending about the same side of the cylindrical surface as the first portion, the second portion further continuing around the opposite side of the cylindrical surface and extending throughout a total angle of substantially 360 and terminating at the end of the structure opposite that where it began, and the third portion beginning at a point closely adjacent the terminal end of the second portion and having a pitch substantially the same as the first portion, the third portion extending around the side of the structure opposite the first portion and through an angle of about 180 and terminating at the end of the structure opposite the beginning point of said first portion.

5. A filament structure as set forth in claim 4 wherein the closely adjacent ends of the first and second portions are connected by an integral bight and the closely adjacent ends of the second and third portions are similarly connected by an integral bight, whereby the portions form a single continuous one-piece strand.

6. A filament structure as set forth in claim 4 wherein support for the filament is provided at the closely adjacent ends of the first and second portions and at the closely adjacent ends of the second and third portions, and at a point substantially midway of the second portion, whereby the portions are restrained from substantial expansion in directions radially of the cylindrical structure.

References Cited in the file of this patent UNITED STATES PATENTS 912,247 Kuzel Feb. 9, 1909 1,658,200 Housekepper Feb. 7, 1928 1,869,998 Cartun Aug. 2, 1932 2,123,015 Marden July 5, 1938 2,141,933 Perrott Dec. 27, 1938 2,156,064 Ronic Apr. 25, 1939 2,173,473 Edwards Sept. 19, 1939 2,297,454 Berger Sept. 29, 1942 2,422,141 Skehan June 10, 1947 2,422,142 Skehan et al. June 10, 1947 2,526,559 Haegele Oct. 17, 1950 FOREIGN PATENTS 121,358 Australia Apr. 12, 1946 

